Many high value uses of organometallic compounds, such as the preparation of semiconductor materials in electronic and optoelectronic applications, require extremely pure organo metallic materials. Organo metallic compounds of Group III elements of the Periodic Table, particularly the lower alkyl compounds of these elements, are extensively used to deposit compounds of their constituent elements on substrates by chemical vapor deposition. For example, gallium arsenide semiconductor layers have been deposited on substrates by combining the vapors of a gallium source such as trimethylgallium with an arsenic source such as arsine at an elevated temperature in the presence of a suitable substrate. Similar processes are used to form other compounds, for example, indium phosphide from trimethylindium and phosphine.
Films of these material may be deposited on surfaces using a variety of techniques including chemical vapor deposition (CVD), physical vapor deposition (PVD), and other epitaxial techniques such as molecular beam epitaxy (MBE), liquid phase epitaxy (LPE), chemical beam epitaxy (CBE) and atomic layer deposition (ALD). CVD processes for example can be used to deposit a metal layer, either at atmospheric pressure or at reduced pressures, by decomposing volatile organometallic precursor compounds, e.g., trimethyl gallium or trimethylindium at elevated temperatures. It is generally accepted that the purity level of the precursor alkyls limits the obtainable purity of the resultant epitaxial layer, which in turn determines the technological usefulness of the resultant device.
This invention relates in particular to the preparation of organometallic compounds suitable for use in vapor deposition and epitaxial growth of metal-containing films.
A number of conceptually simple methods exist for preparing the trialkyl gallium and indium compounds used in the above film forming processes, for example, reaction of metal halides with a Grignard reagent or alkyl lithium in an ether or hydrocarbon solvent, or addition of an organo halide to a molten metal. Thus, trimethyl gallium can be prepared by the reaction of gallium trichloride with 3 equivalents of methyl Grignard or methyl lithium, and trimethyl indium has been formed by the reaction of molten indium metal and methyl chloride. Transalkylation between certain alkyl metal compounds and metal halides is also well known. For example, trimethyl indium has been formed by the reaction of indium trihalide and trimethyl aluminum.
However, there are recognized drawbacks to the existing methods especially when highly pure materials are needed. Some reactions suffer from poor conversions or the formation of products which are difficult to isolate or adequately purify. For example, reaction with a Grignard reagent typically requires a solvent such as ether which is known to tightly complex with, for example, trialkylindium compounds making separation extremely difficult. U.S. Pat. No. 5,455,364 discloses a process for purifying a trialkyl Group III metal compound formed form a Grignard reaction wherein an alkali halide, preferably potassium fluoride is added to the crude product mixture to complex oxygen compounds and distilling the desired product. Also, while many of the more useful organometallic compounds are volatile, e.g., trimethyl aluminum, gallium and indium, so are many of the solvents used in alkylation reactions making separation by distillation difficult.
Along with the issue of purification, transalkylation processes often suffer from the incomplete transfer of alkyl groups from metal alkyl compound to metal chloride. For example, U.S. Pat. No. 3,318,931 discloses a process wherein a threefold excess of trialkyl aluminum is added to gallium trichloride to form the trialkyl gallium compound. That is, only one alkyl group is efficiently transferred from the stating tri-alkyl aluminum to the gallium halide resulting in a mixture of trialkyl gallium and dialkyl aluminum chloride.
U.S. Pat. No. 5,756,786 discloses a method for producing trimethylindium by reacting indium trichloride with a large excess of trimethyl aluminum in the presence of 2 equivalents of potassium bromide in a high boiling hydrocarbon solvent.
U.S. Pat. No. 6,495,707 discloses a continuous method for producing organometallic compounds such as trimethylindium and trimethylgallium by introducing a metal precursor e.g., gallium trichloride and an alkylating agent, e.g., trimethyl aluminum, directly into a distillation apparatus, where upon reaction the volatile trimethyl gallium is distilled away from the remainder of the reaction mixture. An excess of at least 3.5:1 trimethyl aluminum to Gallium precursor must be present in the reaction zone.
Clearly, a process which would allow for the clean transfer of at least two or possibly all three alkyl groups from a trialkyl aluminum to a gallium or indium trihalide would offer a significant improvement in the preparation of these high value trialkyl metal compounds.
Many attempts have been made to improve the efficiency of the transfer of alkyl from, e.g., trimethyl aluminum to gallium trichloride. J. Am. Chem. Soc., vol 84, p 3605-3610 discloses a study of the reaction between triethyl aluminum and gallium or indium trichloride or tribromide. In one experiment, three equivalents of triethyl aluminum is reacted with gallium trichloride in a highly exothermic reaction to provide triethyl gallium in a 38% yield. The subsequent addition of potassium bromide to this initial product mixture and reheating the mixture raised the yield of triethyl gallium to 89% based on gallium. It was postulated that various salts are formed in the reaction. For example, it is believed that Ga[AlEt2Cl2]3 is formed in the initial reaction and that the addition of KBr leads to the presence of K[AlEt2Cl2] in the final product mixture.
While the addition of KBr to the initial reaction product mixture enhances the ultimate yield of trialkyl gallium, a large excess of trialkyl aluminum is still needed due to partial transfer of alkyl groups.
JP 2006/265168 discloses a process for forming trialkyl gallium by heating a mixture of trialkyl aluminum and gallium trihalide either in hydrocarbon solvents or neat. Although it suggests that ratios of trialkyl aluminum to gallium trihalide of 4:1 to 1:1 can be used, all reactions exemplified use about 2.5:1 or ratio of trialkyl aluminum to gallium trihalide. No evidence is provided that good yields or high purity at lower ratios could be obtained.
GB 820,146 discloses a process for forming B, Hg, Ga, Ge, As, Sb and Bi metal alkyls from the corresponding metal chlorides by reacting a mixture comprising a trialkyl aluminum, an alkali metal halide and the metal chloride. The alkali metal salt is believed to from a complex with the aluminum species. Each of the three alkyl groups of the trialkyl aluminum are transferred to the metal chloride and yields of 80 to 90% based on aluminum trihalide are reported, but no data on the conversion of GaCl3 to Ga(alkyl)3 is reported. The disclosure suggests that the reaction may be run in the absence of solvent, although no such reaction is exemplified.
In the production of semiconductors via, e.g., vapor deposition techniques, ultra high purity materials, i.e., materials with level of impurities of <0.1 wt %, preferably <1 ppm, or even <1 ppb are required and the presence of even minute amounts of interfering volatile contaminates is problematic. The presence of residual solvent from the preparation of a trialkyl gallium for example can cause significant difficulty.
One way to avoid contaminants from an organic solvent is to prepare the trialkyl metal compound in the absence of solvent. For example, it has been found by the present inventors that trialkyl gallium or trialkyl indium compounds can be prepared by reacting a tetrahalo gallium salt with a trialkyl aluminum in the absence of an organic solvent. For example, trialkyl gallium compounds are formed by adding a trialkyl aluminum compound to a tetrahalo gallium salt of formula MGaX4 or M(GaX4)2, wherein M is a monovalent metal such as Li, Na, K or Cs or a divalent metal such as Mg or Ca, in the absence of an organic solvent, with high yield and high purity. The tetrahalo gallium salt is fomed by adding a metal halide salt, e.g., a Li, Na or K chloride or bromide, to molten GaCl3. The trialkyl aluminum is added directly to this mixture at temperatures high enough to ensure mixing.
However, during the course of the reaction, efficient mixing can become problematic as various salts and high melting inorganic species are formed. This problem is expected to be more significant when preparing indium compounds as corresponding indium salts have a higher melting point and untenable temperatures may be required.
There remains a need for a highly reliable and efficient route to ultra pure metal alkyls such as trialkyl gallium.
It is believed, as seen in the above cited art, that in the reaction of, e.g., GaCl3 with Al(CH3)3 to form Ga(CH3)3, a variety of organo aluminum halides are formed. In the presence of NaCl for example, these organo aluminum halides would exist as sodium salts such as Na[Al(CH3)2Cl2], Na[Al(CH3)Cl3] and the like. As is common with such inorganic species, the formulae are idealized and variety of more complex salts is always a possibility. In the method described above wherein a trialkyl aluminum is added to a freshly prepared tetrahalo gallate salt, such salts are expected, the distribution of which is determined to a large part by the relative amount of trialkyl aluminum to gallium salts. For example, a large excess of trimethyl aluminum would lead to large amounts of Na[Al(CH3)3Cl], whereas larger amounts of Na[Al(CH3)Cl3] is expected when the amount of trimethyl aluminum is kept to a minimum.
It has been found these salts, or similar salts, can be used as solvents for the transalkylation reaction between metal halides and alkyl metals. Many of these salts are molten at acceptably low temperatures and provide a fluid, non-volatile, ionic liquid medium for the reaction allowing for greater ease in mixing, shorter reaction times and greater flexibility in reactants while avoiding the possible contamination of the product by organic solvents and byproducts, especially as the solvent can be an intermediate that is already believed to be encountered during the reaction.